A shift in the jet stream is responsible for a period of severe weather affecting multiple states and territories in Australia this week and some parts of New Zealand.

Rain and thunderstorms will develop over a broad area of central and southeastern Australia during the next four days. This spell of late-spring storms is being caused by shift in the orientation of a band of powerful upper-level winds called the jet stream.

Our planet’s jet streams are bands of fast-moving air that sit about 10km above the surface. They typically flow from west to east and lie between regions of contrasting temperature.

Image: ECMWF forecast model showing the jet streams flowing over Australia/NZ at the 300hPa level.

There are four main jet streams that affect our weather, two sub-tropical jets and two stronger polar jets in each hemisphere.

When a jet stream flows from west to east, which is called a zonal pattern, weather system tend to be more more transient and generally less severe. However, when a portion of the jet stream bends and flows in more of a north-south orientation, called a meridional pattern, weather systems usually become more stagnant and often more intense. A meridional orientation of the jet stream is usually associated with periods of unusually hot or cold weather as well.

A bend in one of our planet’s jet streams will see a rather strong meridional pattern drift over Australia between Monday and Thursday, before a more zonal flow is restored from Friday.

As a result, widespread rain and storms are on the forecast until Thursday, before more settled weather returns later in the week.

– By Ben Domensino, Weatherzone.com.au, with additional by WeatherWatch.co.nz